<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .demo{
            width: 100px;
            height: 100px;
            background: #000;
            color:#fff;
            padding: 10px;
            border: 5px solid rebeccapurple;
            box-sizing: content-box;
            /* box-sizing: border-box; */
        }
    </style>
</head>
<body>
    <!-- 告诉浏览器怎么计算元素的展示宽高 -->
    <!-- box-sizing:content-box | border-box -->
    <!--  -->
    <div class="demo">
        慕课网 css3 学习分享
    </div>
</body>
</html>